1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and a voltage generation circuit for a liquid crystal display apparatus. More particularly, the present invention relates to a liquid crystal display apparatus for driving a pixel by applying a predetermined modulation voltage corresponding to an illuminated display/unilluminated display to a data electrode, and applying a predetermined programming voltage to a scanning electrode in line sequence, and a voltage generation circuit therefor.
2. Description of the Background Art
In these few years, a liquid crystal display is adapted in the products of various fields such as in the application of AV (Audio and Visual) and OA (Office Automation) owing to the advantages of lightweight, thin and small size, and low power consumption features.
Particularly for those driven by a battery such as a portable equipment, the requirement of reducing power consumption as much as possible is great. Development of a reflective liquid crystal display that does not use a back light of relatively great power consumption and research for reducing the power consumption of the liquid crystal display per se are in progress.
Referring to FIG. 1, a conventional liquid crystal display includes a display panel 1701, a scanning electrode signal driver 1702 for applying a predetermined voltage to a scanning electrode line of display panel 1701 in line sequence, a data electrode signal driver 1703 for applying a predetermined voltage to a data electrode line according to the display information, a voltage generation unit 1706 for generating a voltage to be applied to the Ad liquid crystal display, and a control unit 1705 for providing a control signal to scanning electrode signal driver 1702, data electrode signal driver 1703 and voltage generation unit 1706 for displaying the input information from an input signal line 1704. Voltage generation unit 1706 includes a DC/DC converter 2101 that will be described afterwards.
Referring to FIG. 2, a display panel 1701 includes a plurality of pixels arranged in a matrix. Each pixel includes a liquid crystal display element 1801 connected between a corresponding scanning electrode line (Y1-Ym) and data electrode line (X1-Xn).
Referring to FIG. 1 again, scanning electrode signal driver 1702 includes a shift transistor not shown, and an analog switch. Data electrode signal driver 1703 includes a shift register not shown, a latch circuit, and an analog switch. Scanning electrode signal driver 1702 applies a predetermined voltage to respective scanning electrode lines (Y1-Ym) according to a latch pulse LP and an alternating signal M.
Referring to FIG. 3, scanning electrode signal driver 1702 operates as set forth in the following. In response to latch pulse LP and alternating signal M from control unit 1705, a voltage 1907a of a voltage value VH or a voltage 1907d of a voltage value VL from voltage generation unit 1706 is applied during selected periods 1903 and 1904, and a voltage 1907b of a voltage value VM is applied during a nonselected period for a selected line.
As to an applied waveform 1908 of line Y.sub.i. a voltage 1907a of a voltage value VH is applied to line Y.sub.i during a selected period 1903 in an A frame 1901 in response to latch pulse LP and alternating signal M. In the next B frame 1902, a voltage 1907d of a voltage value VL is applied during selected period 1904 in response to latch pulse LP and alternating signal M. In a nonselected state a voltage 1907b of a voltage value VM is applied to line Y.sub.i.
Application of a direct current component will cause degradation in the characteristics of the liquid crystal material in a liquid crystal display. It is therefore necessary to apply a symmetrical waveform with respect to voltage 1907b of voltage value VM. Therefore, voltage value VL must satisfy VH-VM=VM-VL.
As to an applied waveform 1909 of line Y.sub.i+1, voltage 1907d of voltage value VL is applied to line Y.sub.i+1 during a selected period 1905 of A frame 1901, and voltage 1907a of a voltage value VH is applied to line Y.sub.i+1 at a selected period 1909 in B frame 1902.
Referring to FIG. 4, data electrode signal driver 1703 operates as set forth in the following. In response to latch pulse LP and alternating signal M from control unit 1705 and a data signal D, a voltage 2008a of a voltage value V1 sent from voltage generation unit 1706, and a voltage 2008b of a voltage value VS are applied to the selected line. Here, voltage value V1=2.times.VM.
An applied waveform 2009 to the X.sub.j th data electrode line and an applied waveform 2001 to a position (X.sub.j, Y.sub.i) will be described. A waveform indicated by solid line 2009a is applied to the X.sub.j th data electrode line when the data corresponds to an illuminated display according to latch pulse LP, alternating signal M and data signal D. In the case of a nonilluminated display, a waveform indicated by broken line 2009b is applied to the X.sub.j th data electrode line.
For example, if the liquid crystal display element of position (X.sub.j, Y.sub.i) corresponds to an illuminated display, a voltage of voltage value VS is applied to the X.sub.j th data electrode line during a Y.sub.i line selected period 2003 in A frame 2001, and a voltage of voltage value V1 is applied to the X.sub.j th data electrode line in a Y.sub.i line selected period 2004 in B frame 2002.
The applied waveform to line Y.sub.i is indicated by waveform 2010. The applied waveform to a liquid crystal display element of (X.sub.j, Y.sub.i) is indicated by waveform 2011. Solid line 2011a corresponds to a waveform of an illuminated display, and broken line 2011b corresponds to a waveform of an unilluminated display.
The value of the applied voltage to a liquid crystal display element of (X.sub.j, Y.sub.i) is .vertline.VH.vertline. and .vertline.V1-VL.vertline. in A frame and B frame, respectively, for an illuminated display. For an unilluminatd display, the applied voltage is .vertline.VH-V1.vertline. and .vertline.VL.vertline. in A frame and B frame, respectively. The applied voltage to a liquid crystal display element must be equal in A frame 2001 and B frame 2002. Therefore, EQU .vertline.VL.vertline.=.vertline.VH-V1.vertline.
This gives us -VL=VH-V1. Considering the voltage of voltage value VM,
VH-VM=VM-VL (1)
is achieved by V1=2.times.VM.
Conventionally, a DC/DC converter is used in a voltage generation unit 1706 to generate a plurality of voltages having the above relationship as described in FIG. 1.
When a DC/DC converter 2101 is used as shown in FIG. 5, the input of a voltage of voltage value VD results in the output of a plurality of voltage values VH, V1, VM, VS and VL having the above-described relationship.
The above-described DC/DC converter has a disadvantage that the voltage conversion efficiency at the low current area is extremely low as shown in FIG. 6. This means that only a conversion efficiency of 15-25% can be achieved when the output current is 1-2 mA. Furthermore, a DC/DC converter occupies a definite size since it is a hybrid IC. There was a disadvantage that the size of the mounting substrate must be greater than the size of the DC/DC converter.
The present information society has seen significant development in portable information equipments. The demand for a lighter, thinner, and smaller display with lower power consumption seems insatiable in the field of portable information equipment. However, the usage of a DC/DC converter results in a great loss of power with constraints in the physical dimension, so that these requirements cannot be met. There is also a problem that the cost of the product cannot be reduced since a DC/DC converter is expensive.